U.N. school in Gaza crowded with hundreds of Palestinians seeking refuge from fierce fighting came under fire Thursday, killing at least 15 civilians and leaving a sad tableau of blood-spattered pillows, blankets and children’s clothing scattered in the courtyard.

Palestinian officials blamed Israel for the shelling, which wounded dozens and came on the deadliest day so far of the current round of fighting. However, the Israeli military said the school “was not a target in any way” and raised the possibility the compound was hit by Hamas rockets.

U.N. Secretary-General Ban Ki-moon angrily denounced the attack, saying the killing must “stop now.” But the frantic diplomatic efforts spanning the region were running into a brick wall: Israel demands that Hamas stop firing rockets without conditions, while Gaza’s Islamic militant rulers insist the seven-year Israeli-Egyptian blockade of the territory must end first.

“Many have been killed — including women and children, as well as U.N. staff,” Ban said in a statement, though he did not elaborate and a later U.N. communiqué made no mention of humanitarian workers being among the casualties.

In the aftermath of the attack, a child’s sandal decorated with a yellow flower lay in a puddle of blood, while sheep and cattle belonging to those seeking shelter grazed in the grass nearby. A large scorch mark scarred the spot where one of the shells hit. Dozens of wounded, including many children, were wheeled into a nearby hospital as sirens wailed.

The U.N. said it had been trying to achieve a humanitarian pause in the fighting to allow the evacuation of civilians from the area.

Kamel al-Kafarne, who was in the school, said people were boarding buses when three tank shells hit.

“We were about to get out of the school, then they hit the school. They kept on shelling it,” he said.

It was the fourth time a U.N. facility has been hit in Gaza fighting since the Israeli operation began on July 8. UNRWA, the U.N’s Palestinian refugee agency, has said it discovered dozens of Hamas rockets hidden inside two vacant schools, but U.N. spokesman Farhan Haq said the school hit Thursday in the northern town of Beit Hanoun was not one of them.

The U.N. has also expressed alarm that rockets found in the schools have gone missing after they were turned over to local authorities in Gaza. “Those responsible are turning schools into potential military targets, and endangering the lives of innocent children,” U.N. staff and anyone seeking shelter there, a U.N. statement said.

Fighting was fierce across Gaza Thursday, and at least 119 Palestinians were killed, making it the bloodiest day of the 17-day war. That raised the overall Palestinian death toll to at least 803, Gaza health official Ashraf al-Kidra said. Israel has lost 32 soldiers, all since July 17, when it widened its air campaign into a full-scale ground war. Two Israeli civilians and a Thai worker in Israel have also been killed by rocket or mortar fire.

Israel says the war is meant to halt the relentless rocket fire on its cities by Palestinian militants in Gaza and to destroy a sophisticated network of cross-border tunnels that Hamas is using to sneak into Israel to try to carry out attacks inside communities near the border.

Israel insists it does its utmost to prevent civilian casualties but says Hamas puts Palestinians in danger by hiding arms and fighters in civilian areas.

The Palestinian Red Crescent said Israeli shells hit the U.N. compound. But Israel’s chief military spokesman, Brig. Gen. Moti Almoz, said the military was investigating and it was too early to know if the deaths were caused by an errant Israeli shell or Hamas fire. “We are not ruling out the possibility that it was Hamas fire,” he said.

Another army spokesman, Lt. Col. Peter Lerner, said there had been Hamas fighting in the area.

“We do not target the U.N. We do not target civilians. There was no target in the school. Gunmen were attacking soldiers near the facility. The school was not a target in any way,” Lerner said.

The military had urged the U.N. and the Red Cross to evacuate the school for three days leading up to the shelling incident, Almoz said, adding that there had been an increase in Hamas attacks from the area in recent days.

“Despite repeated calls from the military to the U.N. and international organizations to stop the shooting from there because it endangers our forces, we decided to respond. In parallel to our fire there was Hamas fire at the school,” Almoz said.

The attack was likely to increase pressure on international diplomats shuttling around the region in an effort to broker a cease-fire.

U.S. Secretary of State John Kerry spent the day in Cairo feverishly calling on regional leaders, including Israeli Prime Minister Benjamin Netanyahu as well as the foreign ministers of Egypt, Turkey and Qatar to press for a solution. Like Israel, the U.S. considers Hamas a terrorist organization and will not directly engage with its leaders.

“We still have more work to do. … The tragic incident today and every day just underscores the work we are trying to do and what we are trying to achieve. So we’re going to keep at it,” Kerry said after meeting with the U.N. chief late Thursday. “It’s so imperative to try to find a way forward.”

Ban also urged an end to the fighting. “I am telling … both the Israelis and Hamas: ‘You must stop fighting and enter into dialogue,’” he said. “Whatever grievances you may have, this is wrong. There are many other ways to resolve this issue without killing each other.”

As in years past — most recently in 2012 — the U.S. wants the violence to stop before it tries to negotiate each side’s demands. For Hamas, that includes the release of Palestinian prisoners in addition to an end to the 7-year-old economic blockade imposed by Israel after the Islamic militant group violently seized control of Gaza from the Western-backed government of Palestinian President Mahmoud Abbas.

Egypt tightened its own restrictions last year after the overthrow of a Hamas-friendly government in Cairo and has destroyed many of the cross-border tunnels that had sustained Gaza’s economy, while also being used by the militants to smuggle in arms.

Netanyahu made no reference to the cease-fire efforts in underscoring his determination to neutralize the rocket and tunnel threats.

“We started this operation to return peace and quiet to Israel… and we shall return it,” he said after meeting with British Foreign Secretary Philip Hammond in Israel.

More than 2,300 rockets have been fired at Israel from Gaza since July 8, and the Israeli military says it has uncovered 31 tunnels leading from Gaza to Israel, some of which have been used by Hamas to try to carry out attacks inside Israel. On Thursday, soldiers detained two militants as they emerged from such a tunnel, the army said.

Ominously, meanwhile, violence spread to the West Bank, where thousands of Palestinians protesting the Gaza fighting clashed with Israeli soldiers late Thursday in Qalandia, near the West Bank city of Ramallah. At least one Palestinian was killed and dozens were injured, a Palestinian doctor said.